1. Field of the Invention
The present invention relates to a hot rolling method. More particularly, the present invention relates to a method for rolling metal sheets, in which changes of the surface shape by wear and thermal expansion of the work roll, and a formation of shape defects in the rolled material, such as high spots, edge drop, and an excessive crown, are prevented.
2. Description of the Related Art
A rolling mill in which work rolls of a 4-high or 6-high rolling machine can be moved in the axial direction thereof are described in detail in Japanese Examined Patent Publication No. 50-11859 and Japanese Examined Patent Publication No. 51-7635. This above technique has led to the creation of a new rolling mill, but the manner of use of this machine has not been established at this time.
The following problem is known in the field of rolling. Namely, as described on pages 384 and 385 of Ordinary Hot Rolling (Hot Strip Rolling), Handbook of Iron and Steel, Volume 3(1), Base of Rolling, Steel Sheets, pages 349-482, published May 15, 1980 by Maruzen K. K., the finish work roll, especially the portion through which the strip edge passes, suffers from extreme wear, and thus, if a number of strips having the same narrow width are rolled and a strip having a broad width is rolled thereafter, a so-called high spot (large projection; see FIG. 4) is formed at the edge portion of the strip, which results in edge build-up at the time of re-rolling at the subsequent step and an impairment of the shape.
Japanese Examined Patent Publication No. 59-38842 proposes a method for avoiding this disadvantage by utilizing the above-mentioned rolling mill. This method is characterized in that rolling is carried out by selectively moving pairs of upper and lower flat work rolls every time one or several materials are rolled.
Methods for controlling the edge drop and the crown quantity by using the above-mentioned new rolling mill are proposed in Japanese Examined Patent Publication No. 60-51921 and Japanese Examined Patent Publication No. 60-3881. According to the former method, a pair of upper and lower work rolls having a conical portion (hereinafter referred to as "taper portion") formed on one end in the axial direction are arranged so that the taper portions of the upper and lower work rolls are located opposite to each other, both the side edges of the material to be rolled are located on the above-mentioned taper portion, and rolling is carried out in this state (hereinafter referred to as "one side tapered work roll shift rolling"). According to the latter method, the former rolling method is applied to somewhere in all the stands, of a hot rough rolling or finish rolling mill except the final stand.
However, as is well-known, in these edge drop-controlling methods, the intended effect can be attained at the initial stage, but when several materials are rolled, the effect is substantially lost.
Namely, an effect of preventing the formation of a high spot can be attained according to the technique disclosed in Japanese Examined Patent Publication No. 59-38842, but an effect of controlling the edge drop or the crown quantity is not attained.
According to the above-mentioned one trapezoid roll shift rolling method, rolling is carried out by mutually shifting the upper and lower work rolls so that the side edge of the material to be rolled is located at a predetermined position of the taper portion apart from the taper-initiating point (the joint portion between the conical end portion and the flat portion), and therefore, the work rolls wear at the portion in contact with the material to be rolled and the taper-initiating point shifts with an advance of rolling. Accordingly, the shape of the work rolls is greatly changed after several materials have been rolled, and reduction of the edge drop and the crown quantity becomes impossible, with the result that high spots are sometimes formed and an abnormal crown is formed in the material. These defects cannot be removed by a compensating mechanism such as a bender.
As is apparent from the above description, a rolling technique of preventing a formation of high spots and simultaneously, controlling the edge drop and the crown quantity in rolling of metal sheets, especially hot rolling thereof, has not been established.
In the above-mentioned ordinary hot strip rolling, in order to prevent a formation of high spots, rolling units are constructed so that strips having a broad width are first rolled and strips having a narrow width are then rolled, except in case of levelling materials of which width are changed step-by-step (see FIG. 5) which shows the levelling materials in the shaded portion.
Note, the term "crown quantity" denotes the value obtained by subtracting the thickness of the material to be rolled at a point 25 mm from the side end, from the thickness of the material at the central point of the material. On the other hand, edge drop is a general term indicating the thickness distribution in the vicinity of the side edge portion. As described hereinafter, there is a strong interrelationship therebetween, and therefore, the crown quantity is mainly used as the evaluation parameter hereinafter.